An array of photo pixel sensors detect the intensity of light received by the photo pixel sensors. The photo pixel sensors typically generate electronic signals that have amplitudes that are proportionate to the intensity of the light received by the photo pixel sensors. The photo pixel sensors can convert an optical image into a set of electronic signals. The electronic signals may represent intensities of colors of light received by the photo pixel sensors. The electronic signals can be conditioned and sampled to allow image processing.
Integration of the photo pixel sensors with signal processing circuitry is becoming more important because integration enables miniaturization and simplification of imaging systems. Integration of photo pixel sensors along with analog and digital signal processing circuitry allows electronic imaging systems to be low cost, compact and require low power.
Historically, photo pixel sensors have predominantly been charged coupled devices (CCDs). CCDs are relatively small and can provide a high-fill factor. However, CCDs are very difficult to integrate with digital and analog circuitry. Further, CCDs dissipate large amounts of power and suffer from image smearing problems.
An alternative to CCD sensors are active pixel sensors. Active pixel sensors can be fabricated using standard CMOS processes. Therefore, active pixel sensors can easily be integrated with digital and analog signal processing circuitry. Further, CMOS circuits dissipate small amounts of power.
FIG. 1 shows a prior art array of active pixel sensors. An array of sensors 12 are formed over a substrate 10. An array of color filters 14 is formed over the array of sensors 12. Light passes through the color filters 14 and is received by the array of sensors 12. The sensors 12 typically conduct charge at a rate that is proportional to the intensity of the light received by the sensors 12. Circuitry located on the substrate 10 provides determination of the amount of charge conducted by the sensors 12. Therefore, the sensors 12 provide detection of the intensity of the received light.
The array of color filters 14 includes filters which pass different wavelengths of light. For example, the array of color filters 14 can include blue light, green light and red light filters. The blue light filters only passes blue light, the green light filters only passes green light and the red light filters only pass red light. Generally, each sensor of the array of sensors 12 corresponds with a single color filter of the array of color filters 14. Color detection can be obtained by knowing which sensor corresponds with which type of color filter.
The active pixel sensor array of FIG. 1 is inefficient because up to 70% of the photons within the received light are lost during the filtering process. In addition, the forming the array of color filters is costly and requires valuable substrate 10 area.
It is desirable to have an array of active pixel sensors which provide efficient absorption of photons within light received by the array of active pixel sensors, and be able to detect the color of received light. It is also desirable that the array of active pixel sensors be manufacturable without an array of color filters.